High molecular weight polyesters of terephthalic acid with various diols find extensive use as Dacron, Terylene, Kodel, and Vycron fibers, and Mylar film. These super polyesters, first described in U.S. Pat. No. 2,465,319 to Whinfield and Dickson, in the past have been prepared from dimethylterephthalate, which is trans-esterified with the appropriate diol, such as ethylene glycol, and then polycondensed to form the super polyester. Polyester preparation via dimethylterephthalate has been considered an essential step by reason of the exceptionally high purity requirements imposed on the polyester.
With the advent of improved processes for the manufacture of terephthalic acid, much attention has been directed to obtaining polyesters by direct esterification of terephthalic acid with the diol.
This has manifest advantages of simplicity and economy as compared with the dimethylterephthalate route. As yet, however, there remains serious difficulty in obtaining terephthalic acid of suitable purity. Unless the initial terephthalic acid is virtually completely free from extraneous contaminants, the polyester will have too low a melting point and will be of unsatisfactory color.
It is an object of this invention to reduce the hydrogen used in aromatic carboxylic acid purification and to produce purified products having delta Y values below ten. Another object is to use hydrogen in an amount calculated to be between about ten and seventy-five percent of saturation of the aqueous reaction medium. Using less than full hydrogen saturation of the reaction medium prevents upsets in the catalyst which increase the delta Y values to an unacceptable level. Excess hydrogen usage is a safety risk and can cause reaction upsets which produce product having very high delta Y values, certainly values above ten, the maximum values acceptable to obtain commercially polyesters by direct esterification of terephthalic acid with ethyleneglycol. Additional advantages of the novel process are reduced hydrogen usage and improved catalyst life and activity. A further object is to avoid in our novel process, a gas phase in the catalyst bed, particularly a flash vaporization to the gas phase. This is critical in preventing reactor upsets and increasing the delta Y values above 10 and higher which are unacceptable in the manufacture of polyesters.
It is believed that purified terephthalic acid impurities are carbon particles which give high delta Y values. The main contributor of carbon particles is likely to be the charcoal used to support the nobel metal catalyst. The test for delta Y values is given in the analytic procedures section of this specification. Other impurities are the compound 4-carboxybenzaldehyde, an intermediate formed when terephthalic acid is obtained from the oxidation of paraxylene or other para-disubstituted alkyl benzene, is known to be deleterious with respect to polyester quality. Other impurities are unidentified color bodies, of the benzil or fluoronone structure, usually present as trace byproducts of most terephthalic acid production processes, and yield off-color polyesters. However, any method of purifying terephthalic acid must produce a product having delta Y value below 10.
It is disclosed in U.S. Pat. No. 3,639,465, granted Feb. 1, 1972, that terephthalic acid of a purity suitable for direct esterification with a diol to produce films and fibers may be obtained from the impure terephthalic acid by catalytical hydrogen treatment in polar solvent solution, preferable aqueous solution, of the impure acid under liquid phase conditions preferably by percolating the solution and hydrogen or mixture of hydrogen and inert gas through a bed of catalyst particles having noble metal hydrogenation catalyst. The improvement afforded by our novel process over U.S. Pat. No. 3,639,465 is the contacting of the catalyst with the solution of aromatic polycarboxylic acid and hydrogen dissolved in the solvent with a noble metal-containing catalyst so that no gaseous phase is present in the catalyst bed and that the liquid phase has a hydrogen concentration of about 10 to 75 of saturation, preferably about 40 to 60 percent.
When any gas or mixture of gas and vapor comes in contact with a pure liquid (that is liquid having no dissolved gas) the gas or soluble component of a gaseous mixture will dissolve in the liquid. If this contact is maintained for a sufficient length of time, equilibrium is attained. Regardless of the duration of contact between the liquid and gas, after equilibrium is reached no more gas will dissolve into the liquid phase. The liquid is then said to be saturated with the particular gas at the given conditions. For the given conditions saturation is the highest equilibrium concentration of gas which will dissolve in the liquid. When less than this equilibrium amount of gas is dissolved in the liquid the situation is called partial saturation. Percent of saturation is 100 multiplied by the ratio of the amount of gas dissolved to the amount of gas which would be dissolved at equilibrium where the amount of dissolved gas is measured in volume units of gas per mass of liquid solution at reference conditions. For example 100 percent of saturation is about 0.6 cc of hydrogen per gram of solution at a temperature of about 525.degree. F. and a hydrogen pressure of about 150 psia above an aqueous solution pressure containing about 9.1 weight percent terephthalic acid. When the aqueous solution contains about 23.1 weight percent terephthalic acid, 100 percent of saturation increases to about 0.77 cc of hydrogen per gram of solution. The reference state for the hydrogen volumes is 32.degree. F. and 1 atms.
In the prior art, including U.S. Pat. No. 3,639,465, the hydrogenation is conducted in such a manner that the reactor feed is fully saturated and an excess gaseous phase is present in the reactor. The disadvantage of prior art procedure is that the palladium charcoal catalyst tends to be crushed and small carbon particles are introduced into the final purified terephthalic acid product. This carbon contamination of the purified terephthalic acid is measured as a delta Y. For example fine carbon particles added to purified terephthalic acid at a level of only 1 ppm increases delta Y value to more than twenty. A commercially acceptable product should have a delta Y below 10. When the hydrogenation of the terephthalic acid impurities are conducted in the manner according to this invention, wherein hydrogen saturation of the reaction medium is kept at about 10 to about 75 percent of saturation, delta values below 10 are obtained. In the best mode for conducting our process the hydrogen concentration of about 40 to about 60 percent of saturation in the liquid is maintained without having any gaseous phase present in the catalyst.
According to the present invention, there is provided a process for purifying aromatic polycarboxylic acid produced by liquid phase catalytic oxidation of polyalkyl aromatic hydrocarbons to remove carbon particles and undesirable aldehyde and other impurities comprising the steps of forming an aqueous solution of said acid containing said impurities in water, contacting said solution and hydrogen at about 10 to about 75 percent of saturation of the liquid medium with a noble metal-containing catalyst, and recovering purified acid having delta values below 10. This process is particularly well suited for purification of aromatic dicarboxylic acids, for example, purifying crude terephthalic acid. The recovering of purified acid is conveniently effected by crystallizing the acid from the hydrogen treated aqueous solution, wherein the hydrogen in the reaction medium is in the range of about 10 to about 75 percent of saturation of the aqueous solution. The noble metal-containing catalyst suitably comprises charcoal having supported thereon 0.01 to 1 percent by weight of the noble metal. Preferably the charcoal support is one having a surface area in the range of about 1,000 to 2,000 square meters per gram. Particularly desirable noble metals for use as the catalyst are platinum and palladium, preferably palladium.
The present invention also provides a method of purifying crude aromatic polycarboxylic acid obtained by catalytic liquid phase oxidation of a polyalkyl aromatic hydrocarbon with molecular oxygen in the presence of a heavy metal oxidation catalyst said aromatic hydrocarbon having at least two nuclear alkyl hydrocarbon substituents whose carbon attached to the nuclear aromatic carbon has at least one hydrogen atom, which crude aromatic polycarboxylic acid has an aromatic polycarboxylic acid content of at least 99.0 percent by weight, preferably 99.5 weight percent, and which has as its principal impurity a carboxy aromatic aldehyde corresponding to said aromatic polycarboxylic acid, which method comprises forming an aqueous liquid solution of said crude acid in water, contacting said solution in the liquid phase at a temperature in the range of about 450.degree.-600.degree. F. with hydrogen at about 10 to about 75 percent of saturation of the reaction medium with a noble metal-containing hydrogenation catalyst for a time sufficient to effect substantial reduction of said aldehyde and without introducing carbon impurities and recovering purified acid having delta Y values of less than 10.
According to a preferred embodiment of the present invention there is provided a method of producing fiber-grade terephthalic acid from crude terephthalic acid obtained by catalytic liquid phase oxidation of paraxylene with molecular oxygen in the presence of a heavy metal oxidation catalyst, which crude terephthalic acid has a terephthalic acid content of at least 99.0 percent by weight, preferably 99.5 weight percent, and which has as its principal impurity 4-carboxybenzaldehyde, which method comprises forming a solution of said crude terephthalic acid in water, contacting said solution in the liquid phase at a temperature in the range of about 450.degree. to 600.degree. F. and hydrogen mixture at about 10 to about 75 percent of saturation of the reaction medium with a noble metal-charcoal hydrogenation catalyst for a time sufficient to effect substantial reduction of said aldehyde to para-toluic acid and recovering fiber-grade terephthalic acid by crystallization having delta Y values below 10.
When terephthalic acid, or other aromatic discarboxylic acid to be employed in super polyester production, is purified by the process of the present invention it is said to be of "fiber-grade" quality. The term "fiber-grade" does not denote a quantitative degree of purity, but rather describes a terephthalic acid which is sufficiently free from 4-carboxybenzaldehyde and has a delta Y value below 10 to yield a super polyester upon direct esterification with a diol which is satisfactory for the intended purpose.
The process of the present invention has particular application to purification of terephthalic acid produced by the liquid phase air (molecular oxygen) oxidation of paraxylene using a heavy metal and bromine as catalyst as described in Saffer et al. U.S. Pat. No. 2,833,816. The process of the present invention may also be used to advantage for purification of terephthalic acid from other processes for the catalytic liquid phase oxidation of para-dialkylbenzenes with molecular oxygen in the presence of heavy metal oxidation catalyst, also promoted with acetaldehyde or methyl ethyl ketone, for the terephthalic acids produced by these oxidation processes also contain 4-CBA impurity. Terephthalic acid from any source which contains 4-carboxybenzaldehyde and which is yellowish in color, can be converted to fiber grade terephthalic acid by the process of this invention.
The process of the invention is conducted at elevated temperature and pressure while the terephthalic acid or other aromatic polycarboxylic acid is dissolved in an inert polar solvent. Examples of suitable solvents include the lower molecular weight alkyl carboxylic acids and water, with water being the preferred solvent. By reason of its low solubility in water, terephthalic acid requires either large volumes of water or high temperatures in order for the desired terephthalic acid production quantity to be put into solution. For reasons of economic equipment design and process operation, it is therefore desirable to conduct the process within the range of about 392.degree. to about 700.degree. F., although lower or higher temperatures may be used in particular circumstances. The most advantageous temperature range is about 440.degree.-575.degree. F., e.g. 464.degree.-550.degree. F. The quantity of water needed to dissolve the terephthalic acid at various temperatures may be estimated from the table below:
TABLE I ______________________________________ Terephthalic acid, Temperature, .degree.F. g./100g.H.sub.2 O for solution ______________________________________ 1 365 5 401 10 468 20 498 30 522 ______________________________________
For the purification of crude terephthalic acid aqueous solution temperatures in the range of 450.degree. to 600.degree. F. are preferred because these solutions carry more than 5 pounds of the acid per 100 pounds of water.
Pressure conditions for the process of this invention depend upon the temperature at which this process is conducted. Since the temperature at which significant amounts of the impure terephthalic acid may be dissolved in water are substantially above the normal boiling point of water, and since the hydrogenation section of the process of this invention is to be carried out with the solvent in the liquid phase, the pressure will necessarily be substantially above atmospheric pressure.
Hydrogen treating time, or space velocity, will depend on the initial terephthalic acid purity, that is, the amount of impurity to be reduced, on the desired fiber-grade specifications imposed on the purified terephthalic acid, and on other conditions of the hydrogenation such as for example, catalyst activity. Ordinarily a treating time, i.e. contact time with the catalyst, within the range of about 0.001 to about 10 hours, advantageously about 0.01 to 2 hours, will suffice for most operations. Although treating time is not a critical variable, it must be taken into consideration with regard to the aforementioned severe hydrogenation and its side effects.
The hydrogenation catalyst required for the process of this invention to convert the aldehyde carbonyl group on the 4-carboxybenzaldehyde (4-CBA) at least to a methylol group, e.g. to convert the 4-CBA to p-methylol benzoic acid, and to destroy, or otherwise render innocuous, other impurities present (e.g. those of benzil and fluorenone structure) in the feed terephthalic acid is preferably a Group VIII noble metal, preferably platinum and/or palladium, supported on adsorbent, high surface area charcoal. A wide variety of catalysts have been found efficacious, and while carbon-supported noble metals are outstanding, reference may be made to any of the standard texts on hydrogenation or catalysts for alternative materials which are catalytically effective under aqueous phase hydrogenation conditions. It must be kept in mind, however, that the catalyst used must be one which is useful for effecting the hydrogenation under mild hydrogenation conditions as defined herein. Numerous catalysts are listed, for example, in Kirk and Othmer's "Encyclopedia of Chemical Technology" (Interscience), particularly the chapters on Hydrogenation and Catalysts; Emmett's "Catalysis", (Reinhold), particularly Volumes IV and I on Hydrogenation; Lohse's "Catalytic Chemistry" (Chemical Publishing Company), particularly the sections on Group VIII Metal Catalysts; and such patents as U.S. Pat. Nos. 2,070,770 and 2,105,664. Illustrative catalysts include the Group VIII Hoble Metals Ruthenium, Rhodium, Palladium, Osmium, Irridium, and Platinum, advantageously extended on a support such as activated carbon, e.g. adsorbent charcoal.
The noble metal hydrogenation catalyst for use in the inventive process must have sufficient hydrogenation activity to convert the aldehyde carbonyl group on the 4-carboxybenzaldehyde at least to a methylol group, e.g. p-methylol benzoic acid, and to destroy, or otherwise render innocuous, other impurities present in the feed terephthalic acid. Noble metal supported on adsorbent charcoal in the amount of 0.01-1.0 weight percent, based on total catalyst, is suitable as the hydrogenation catalyst. Advantageously, nobel metal contents in the range of about 0.05-0.5 weight percent may be used, with about 0.3-0.7 weight percent being the preferred noble metal content for use in trickle beds of catalyst. The higher noble metal contents tend to produce over-hydrogenation while the lesser amounts suffer some loss in hydrogenation activity as compared with catalysts of the preferred noble metal content.
The adsorbent charcoal support for the noble metal may be any such support which has sufficient mechanical strength and surface area. It has been sound that palladium-charcoal catalysts having a palladium content in the preferred range of 0.3-0.7 weight percent and also having a very high surface in the range of about 1000-3000 square meters per gram of catalyst are particularly well suited for use in the present invention.